Demodulator circuits



Oct. 31, 1944. c EVERETT 2,361,616

.DEMODULATOR CIRCUIT Filed Aug. 51, 1942 7 LP. AMPL. I I

I J TO A.F. W :3 L AMPLIFIER o m 4 PRIOR LIL 2o 1 TAGE 2| 'E la 1;

A.V.C

1 TO A F I NETWORK 40 5 41 TO l.F.

AMPLIFIER REFLEX 36 r-l6 AVC.

INVENTOR FREDERICK C EVERETT ATTORNEY Patented Oct. 1'51,v 1944 DEMODULATOR CIRCUITS Frederick C. Everett, Brecksville, Ohio, assignor to Radio Corporation of America, a corporation of Delaware Application August 31, 1942, Serial No. 456,739 I 5 Claims.

My present invention relates to demodulator circuits for radio receivers, and more particularly to novel and improved types of diode detector circuits.

In the past the diode detector has become quite popular, because of its linearity and improved performance in providing automatic volume control (AVC) voltage. It does, however, possess the serious disadvantage of loading its signal input transformer secondary. Hence, most, or all, of the gain and selectivity contributed by the diode detector input transformer is lost. Of course, other types of detectors have been proposed. For example, the plate rectification type of detector and infinite impedance (also known as the degenerative plate rectification) detector have been used.' However, they both fail to provide sufficent AVC voltage. This means that a separate radio frequency amplifier and diode rectifier must be used to furnish adequate AVC bias. It must, also, be noted that the plate rectification detector is not as linear in its performance as the diode detector.

It is one of the main objects of my invention to provide a detection system giving the advantages of diode rectification; the essential feature of the system comprising the shunting Of a diode across an impedance in the cathode circuit of a grid-controlled space discharge device.

Another important object of my invention is to provide a grid-controlled electron discharge device which has an impedance in its space current path, and at least one diode being shunted across the impedance to provide rectified signal voltage; the diode thereby exercising no loading on the signal input circuit connected to the'grid of said electron discharge device.

Another object of my invention i to provide a double diode-triode circuit wherein a radio frequency signal input circuit is connected to the triode input, while the diodes are separately shunted across an inductive impedance in the triode cathode circuit thereby providing separate signal demodulation networks.

Yet another object of my invention is to provide a double diode-triode network wherein the triode has an inductive impedance in its cathode circuit, the signal input circuit being connected to the input electrodes of the triode, one diode being in shunt with the impedance to provide AVC bias, while the other diode is connected in shunt with the impedance to provide audio frequency voltage which is refiexed back to the triode input circuit whereby the triode may function as an audio amplifier.

The novel features which I believe to be characteristic of my invention are set forth with particularityin the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically two circuit organizations whereby my invention may be carried into effect.

In the drawing-- Fig. 1 shows a circuit employing the invention,

Fig. 2 shows a modification.

Referring now to the accompanying drawing, wherein like reference characters in the two figures designate similar circuit elements, in Fig. 1 my invention is shown applied to the second detector network of a superheterodyne receiver. Whil I have shown, for the purpose of illustration, the invention embodied in a superheterodyne receiver of the'broadcast type, it is to be clearly understood that the invention'is applicable to any frequency range and to any desired type of receiver. may be applied to the demodulator network of a tuned radio frequency amplifier type of receiver. For the purposes of illustration let it be assumed that signals are being used in the broadcast band of 550 to 1700 kilocycles (kc), and that the operating intermediate frequency (I. F.) is of approximately 455 kc. This latter value is purely illustrative, and any other frequency value may be employed.

Those skilled in the art of radio communication are fully acquainted with the fact that the received modulated carrier energy is first collected at an antenna, then amplified in one or more stages of tuned radio frequency amplification at carrier frequency. Thereafter, the amplified signal energy is converted to I. F. energy, and the latter may be amplified in one or more stages of I. F. amplification. The numeral I denotes the I. F. amplifier stage which feeds the control grid 2 of multiple function tube 3.

The tube 3 is shown by way of example as one of the double diode-triode type. It is to be clearly understood that the invention is not limited to the inclusion of the diode electrodes in the same tube envelope as the triode electrodes.

Furthermore, in place of the triode section there may be used a screen grid, or a pentode section.

However, since the double diode-triode is fairly commonly used, it is shown by way of illustration. The tube 3 has its cathode 4 connected to ground through an inductive impedance 5.

For example, the invention This impedance may be a simple inductance coil. The triode section of tube 3 consists of cathode 4, input grid Zgand plate 6.

The plate is connected to the +3 terminal of the" direct current supply source. The condenser l connects the plate to ground, and functions to by-pass I. F. and audio frequency currents. The grid 2 i connected to the high alternating potential side'of the resonant input circuit 8. This circuit consists of the secondary winding 9 of transformer ili and the shunt tuning condenser ID. The low potential end of secondary winding 9 is connected to a negative biasing voltage source through a resistor M which acts as a filter resistor. Condenser l2, which. is sufiiciently large to by-pass I. F. and audio currents, is connectedfrom the upper endof resistor H to ground.

The transformer ID has its primary winding shunted by a tuning condenser, and the fixedly resonant circuit formed thereby may be included in the plate circuit of the I. F. amplifier I. The cathode d additionally provides electrons to the auxiliary diode anodes i3 and it. Thus, a pair of separate diodes are provided. The anode i4 is connected to ground through the load resistor I5. It will, therefore, be seen that the diode d-M has its space current path arranged in series with the impedance 5 and the load resistance [5. The rectified voltage developed across resistor i5 is employed for AVC bias. For this purpose, the lead it is connected to the anode end of resistor i5 through a filter resistor II which removes all pulsating voltage components.

The lead It, which is the AVG lead, may be connected to the control grids of the various prior high frequency amplifiers which are to be controlled in gain. I. F. bypass condenser 16 is connected from lead IE to ground. AVC bias is to be understood as supplied through filter resistor I8 to the control grid of I. F. 'amplier l.'

Thoseskilled in the art are fully acquainted with the purpose and functions of the AVG connection. Briefly, the AVG bias varies directly in magnitude with the carrier intensity. The gain of the prior controlled amplifiers is varied in a sense such as to maintain substantial uniform carrier amplitude at input circuit 8, regardless of wide carrier amplitude variations at the receiver antenna circuit.

Anode i3 is connected load resistor 20, the latter being shunted by condenser 2| which is an I. F. by-pass condenser. The rectified voltage developed across resistor 29 is utilized to provide the audio voltage to a following audio frequency amplifier stage. One or more audio frequency amplifiers may be used, and any desired type of reproducer will follow the last audio amplifier. The gain through the triode section of tube 3 is unity. Hence, no amplification is obtained through that portion of the tube. However, the actual overall gain goes up, because of the reflected load into the plate of the preceding I. F. amplifier which is higher. This follows from the fact that the triode section of the tube 3 functions as an infinite impedance circuit. This means that the selectivity is, also, considerably increased.

The presence of the two diode anodes l4 and I3 in the double diode-triode type of tubes makes it easy to utilize one to deliver the audio voltage and the other to supply the AVG bias. This eases the filter requirement, aids the stability of the receiver, and avoids the loading of the diode audio circuit which would cause distortion. Howto ground: through its ever, it is to be understood that both audio and AVC voltages can be obtained from a single diode anode, if only the one is available within the tube envelope. When necessary the cathode coil 5 should be shielded. I

Since cathode bias cannot be used without biasing the diode anodes, a source of auxiliary negative bias is provided for grid 2. This may be present in the receiver without any additions. Of course a special biasing cell may-be used, or a small dry cell. The filter resistor in the nega-. tive power supply lead will provide the necessary few volts of negative bias. If desired, a, part of the AVC'bias maybe used for the same purpose, although, this is not an advantageous expedient. Of course, the diode may be provided by a separate 6H6 type of tube Without change in the circuits shown in Fig. l. In this present circuit arrangement the diodes are prevented from loading the input transformer [0, because the triode grid 2 is kept at a negative potential which is greater than the incoming peak signal voltage. Considering each diode detector during the time each anode is positive, electrons fiow to the anode thereby representing a flow of current thus causing the tuned circuit to deliver power. The latter condition represents a loading of the tuned circuit 8. This is also true in the case of the grid leak type ofdetector, where the grid is allowed to attract electrons by becoming positive and thereby drawing electron current. Plate rectification detectors and degenerative plate circuit detectors have sufiicient negative bias on the grid so that the grid is prevented from going positive. In the present circuit each diode is'shunted across the cathode coil, thus the tuned circuit 8 is not loaded. However, the grid voltage is varied at a radio frequency rate, and the plate current variations correspond to the grid voltage excursions. The plate current flows through the cathode impedance, which provides excitation for each diode.

If, economy of equipment is necessary to decrease cost, or increase portability, it is possible to refiex audio voltage into the grid circuit of the triode section. In this way the triode section is concurrently used as an audio amplifier. Referring to Fig. 2, diode anode l4 provides the AVG rectifier as in Fig. 1. The audio voltage taken ofi across resistor 20 is refiexed over lead 30 to the low alternating potential side of signal input circuit 8. The triode section of tube 3 now functions as an audio amplifier, and amplified audio voltage is taken off across audio output resistor 40. The latter is shunted by condenser M which has a high audio impedance, but low I. F. impedance. The load resistor 40 has its lower end connected to the +3 terminal through a filter resistor 40' bypassed by condenser 4|.

The reflex portion of the circuit is designed to avoid overloading of the tube when maximum I. F. and audio voltages are applied to grid 2 at the same time. Overloading usually occurs in that case since the straight line portion of the negative grid voltage region of the tube characteristic is limited. However, in this type of reflex circuit, by using a tube with a low mu triode section such as is found in a 6R7 type tube, a rather larger linear portion is available.

The reactive magnitude of the cathode coil 5 will vary with the value of employed I. F. The

inductance of coil 5 will be about the same as the inductive magnitude of the coil 9 for the operating frequency. Too large a value may produce oscillation, although some negative resist-' ance may assist the gain and selectivity Where allowable. The input transformer II] should be designed to work into a high impedance, and then the design can be for maximum selectivity and/or maximum gain. a

While I have indicated and described two systems for carrying my invention into efiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a tube having at least a cathode, input electrode, output anode and'a diode section, a modulated carrier input circuit connected between the input electrode and cathode, an untuned inductive impedance connected from cathode to ground, said diode section being connected in shunt to said impedance, means for negatively biasing said input electrode to a value in excess of applied carrier peaks, and means for developing voltage from rectified modulated carrier current flowing through the diode section.

2. In a demodulator system for modulated carrier current, a tube having at least a cathode, control grid, plate and a first and second diode section, a carrier input circuit between the grid and cathode, solely an unbypassed inductance coil connected from said cathode to ground, said first diode section being connected in shunt with said inductance, a resistor in circuit with said first diode section to develop rectified carrier voltage, said second diode section being connected in shunt with the inductance, a second resistor in circuit with said second diode section to develop a second rectified carrier voltage, means to negatively bias said grid to a Value in excess of applied carrier peaks, and means for applying one of the rectified voltages to said control grid for reflex action.

3. In a demodulator system for modulated carrier current, a tube having at least a cathode, a control grid and plate, a carrier input circuit consisting of a coil and condenser providing a parallel tuned circuit between the grid and cathode, an untuned inductance connected from said cathode to ground, said coil and inductance being substantially equal at the carrier frequency, a first diode connected in shunt with said inductance, a resistor in circuit with the diode to develop rectified carrier voltage, automatic volume control means connected to the resistor, a second diode connected in shunt with the inductance, a second resistor in circuit with the second diode to develop an audio voltage, means to bias said grid to a negative value in excess of carrier peaks, means consisting of connections from the second resistor to said control grid and cathode for applying said audio voltage to said tube for reflex action.

4. In a demodulator system for modulated carrier current, a tube having at least a cathode, a control grid and plate, a carrier input circuit consisting of a coil and condenser connected to provide a parallel resonant circuit between the grid and cathode, an untuned inductance connected from said cathode to ground, said coil and inductance being substantially equal in reactive value at the operating carrier frequency, a first diode connected in shunt with said inductance, a resistor in circuit with the diode to develop rectified carrier voltage, automatic volume control means connected to the resistor, a second diode connected in shunt with the inductance, a second resistor in circuit with the second diode to develop an audio voltage, connections from said second resistor to said control grid and cathode for applying said audio voltage to said tube for reflex action, and both said diodes having the electrodes thereof in the same tube envelope as the cathode, control grid and plate.

5. In combination with a grid-controlled electron discharge device of the type including at least a control grid, cathode and plate, a signal input circuit consisting of a coil and condenser providing a parallel resonant circuit connected between said grid and cathode, an untuned inductive impedance connected to the cathode and being located in the space current path of said device, said coil and inductive impedance being substantially equal in magnitude at signal frequency, at least one rectifier being connected in shunt with said impedance, a load impedance connected in circuit with the rectifier to develop rectified signal voltage thereacross, and connection means from said load impedance to said grid and cathode for reflexing the rectified voltage to said device for amplification thereby.

FREDERICK C. EVERETT. 

